Finger-joining short pieces of lumber together at their ends to make longer pieces of lumber is becoming ever more important as attempts are made to more efficiently use the lumber resource. In finger joining, matching interleaving fingers are cut into the ends of the members to be joined, adhesive is applied to the surfaces of the fingers, the parts are joined and longitudinal compressive force is applied while the adhesive cures. In some cases, short sections of lumber are left over as a by-product of production trimming operations. In other cases, the "shorts" result from cutting out naturally occurring defects. In still other cases longer lumber than the standard lengths available from normal sawmill operations is desired. In any event the economic value of short pieces is low, even if the inherent wood quality is high. Finger-joining the shorts into longer pieces is one method for increasing their value. In some applications, such as for studs in walls, finger-joined lumber may actually be more valuable than the competing one-piece studs they replace because the finger-joined product is generally straighter.
While the application of finger-joined lumber in studs is an important one, finger-joined lumber is also being used in more structurally demanding applications such as in trusses where large tensile or bending loads may be applied. By cutting out strength reducing characteristics such as knots and finger-joining the remaining short pieces, long pieces can be obtained without the strength reducing characteristics. But, this requires that the finger joints themselves be structurally sound. In these applications, finger-joined lumber must have sufficient quality to withstand design loads plus some margin of safety. As a result, interest has developed concerning methods of testing the finger joints. Finger joints can also be used to join other wood products including laminated veneer lumber and panel products as well as lumber.
In this disclosure, the term wood refers to any product made from wood fiber which includes wood timbers of all sizes, reconstituted wood products such as laminated veneer lumber and panel products. Finger joint quality refers generally to the structural quality of the finger joint. Usually, this will mean bending or tensile strength, but it could refer to any of the other structural properties such as compression strength, shear strength or bending modulus of elasticity.
Off-line tests of finger joint quality can be done on a sampling basis. The reasoning is that poor finger joints result from something going wrong in the process. Consistent process problems usually will be detected by testing production samples. Off-line tests may consist of applying increasing tensile loads to the wood through the finger joint, measuring the ultimate tensile load at failure and inspection of failed areas in the finger joint. Alternatively a fixed proof load equal to some factor above the design load can be applied to the wood and through statistical means, the number of failures can be used to define finger joint quality. However, any off-line quality control testing of finger joints which does not test all of production leaves some risk of missing significant problems in the joints. Production-line testing of every joint is a better approach.
Production-line proof load tests are possible wherein either bending or tensile stresses are applied. Both types of equipment have been proposed for use and have been put to work in the production-line (Faoro, Oscar, 1985. Proof Loading to Establish the Integrity of Structural Finger Joints. Fifth Nondestructive Testing of Wood Symposium. Pullman, Washington. and Eby, R. E., 1981. Proofloading of Finger-joints for Glulam Timber. Forest Prod. J. 31(1):37-41.), although technical arguments have been made in favor of tensile proof load testing (Logan, James D., 1982. Proof Load Testing Finger End-Joined Lumber Tension or Bending?. Metriguard Inc. Pullman, Washington.). Production-line proof load testing in either bending or tensile modes requires considerable space and leads to other problems as well. Among them are the handling of broken wood product as a result of applying the proof load and the continuity of production when a failure occurs. Perhaps the most severe problem, particularly in the case of bending proof testing, is the risk of overstressing the finger joint because of incomplete adhesive cure at the point in the production-line where testing is most convenient and hence most often performed.
The present invention avoids the problem of overstressing the finger joints by nondestructively passing stress waves in a transverse direction through the finger joints and processing the resulting signals to give a measure of finger joint quality. Further, the apparatus can be implemented more efficiently and conveniently in much less space in the production line than can proof testing equipment. This nondestructive method will not eliminate the need for tension proof testing in those cases where stress testing of every piece is necessary; rather it gives a predictive measure of finger joint quality. In many cases that will be a sufficient indication of finger joint quality. In other cases the prediction can be used to adjust the amount of tensile proof load applied and hence determine the final grade of the product, assuming it survives the proof load. The economics of the process of predicting strength from a correlated variable and adjusting the proof load to fit has been studied (Bechtel, F. K. 1983. Proof Test Load Value Determination for Maximum Economic Return. Forest Prod. J. 33(10):30-38.).
Testing of wood and other materials by measuring stress wave propagation time (or velocity) by either acoustic or ultrasonic means has long been used. Commercial examples include the Metriguard Model 239A Stress Wave Timer and the Metriguard Model 2600 Ultrasonic Veneer Grader, U.S. (Logan, James D., U.S. Pat. No. 4,201,093). Other things being equal, stress waves pass through structurally higher quality materials faster than they do through poorer quality materials. This is the basis of the Model 2600 Veneer Grader which grades according to the propagation time of an ultrasonic stress wave from one end of a veneer sheet to the other. Recently, in an attempt to nondestructively quantify the quality of finger joints, the present invention was conceived. The invention involves passing stress waves transversely through the finger joints with a modified version of the Model 2600 Veneer Grader transducer wheels. These wheels had previously been modified to focus the ultrasonic energy primarily in a radial direction of the wheel instead of in the axial direction as required for the Veneer Grader. The wheels were modified for use in Metriguard's research into delamination of reconstituted wood products such as laminated veneer lumber (LVL). In that research, the modified transducer wheels are placed above and below the product, and a measure of the propagation time and/or attenuation of the stress wave is obtained. That research has been going on for several years with various signal processing methods employed. Similar methods have been employed by others (Shearer, et al. U.S. Pat. No. 4,750,368 and Shearer, et al. U.S. Pat. No. 4,856,334). In these patterns, methods are described wherein ultrasonic energy is introduced by a transducer on one side of a composite panel and received by another transducer on the other side of the panel. Received signal voltage based on amplitude or ringdown count, panel temperature and panel thickness are used to determine the quality of panel bonding. None of these methods have been used or have been suggested for use in the detection of wood finger joint quality or of lumber quality by passing stress waves transversely through the joints.
The present invention uses stress waves in the estimation of finger joint quality to avoid overstressing the joint, and new methods are disclosed for using the stress waves in the determination of wood finger joint quality and of lumber quality.